This invention relates generally to machinery for screening paper-making pulp and, more particularly, to a screening apparatus having an enhanced rotor for promoting screening efficiency together with power conservation.
The Pulp and Paper Industry uses pressure screens to separate undesirable materials from usable fiber in the Industries' various processes. The typical pressure screen has a cylindrical screen plate with apertures in it. Inside of that is a central rotating element, the rotor, to provide pressure pulses that function to “clean” the surface of the screen plate and provide a motive force to move fibers through the plate. The screen rotors are characterized by the speed of rotation at the outermost point of the rotor (tip speed, usually expressed as meters/sec) and the frequency with which a rotor element passes a point on the screen (Hertz). The design of the rotor element controls the pulse generation function of the rotor.
Different types of pulp from different manufacturing processes require variations of the screening technique. For the purpose of this invention, the class of fibers produced by mechanical means will be considered. Examples of some of the processes which produce this type of fiber are stone groundwood, mechanical refiner groundwood, thermo-mechanical and chemi-thermo-mechanical pulps. In each of these processes the primary role of the screen is to separate the refined fibers from larger fiber bundles, called “shives” in the industry. The separated shives are recycled for additional refining. Some of the processes also desire separation of some of the longer fibers from the shorter fibers by the same mechanism of screening.
When screening mechanical pulps, the short flexible fibers that need to pass through the screen easily make the turn into the screen apertures. The longer less flexible fibers that require more refining action before they are ready to pass through the screen, need to be lifted away from the screen apertures and removed for further processing.
An example of current technology could be called a cage type rotor. A cage type rotor uses axial bars running close to the surface of the screen, and may have either notches in the trailing edge or small vanes on the surface of the element where its clearance with the screen is becoming greater. The notches or small vanes are angled toward the bottom, or the reject end of the screen. The element is typically called a “foil” and has a blunt leading edge and is triangular or square in cross section. These foil elements are suspended from a relatively narrow central core of the rotor, leaving the majority of the space inside the screen as void space, or space that is taken up by the fiber suspension. These rotors may also have a vertical plate either attached to the rotor arms, or extending from the central core and between the rotor arms extending to the foil elements.
Each foil member extends axially for the full length of the screen. The cage type rotor generates pulses, which sweep around the circumference over the full length of the screen with every revolution of the rotor. Such rotors consume excess power due to stirring action on the pulp residing inboard of the foil members. This power is wasted because it does not contribute sufficiently to the screening action.
To reduce the magnitude of the effects described above, many machines are made with closed rotors, that is, rotors having a full cylindrical surface on which bumps and depressions are directly attached without support arms to generate localized pressure pulsations. Depending upon their specific geometries, these may offer lower specific power consumption than cage rotors; and, because the bumps and depressions are distributed over the rotor surface, the pressure pulsations are distributed about the screen surface and do not concentrate alternating stresses along the aperture pattern
One improvement to the cage and closed type rotors provides a large diameter hub on which the hydrodynamic foils are each mounted on short support arms to reduce the volume of the screening chamber and to reduce specific power consumption. This configuration can also be used to control flow patterns within the screening zone of the screen body.